The present invention relates to a communication control method and apparatus for reducing the number of messages exchanged among data processing terminals, such as VTRs, computers, or video displays, for example.
When a plurality of terminals, such as VTRs, are connected to a common data bus, the VTRs exchange commands among each other. In such a network, when a command packet of command data is received by a terminal, the receiving terminal provides a response packet to the transmitting terminal; this response packet informs the transmitting terminal that the receiving terminal has received the command. The terminals in the network generate two types of commands: commands that can be performed simultaneously at any one particular terminal and commands that cannot be performed simultaneously at any one particular terminal. The first type of command is referred to as a simultaneous command, and the second type of command is referred to as a non-simultaneous command. A simultaneous command can be performed alone, or simultaneously with any other command to be performed at that terminal, including a non-simultaneous command. A non-simultaneous command, however, is never performed simultaneously with another non-simultaneous command. A non-simultaneous command is performed at the terminal either alone or simultaneously with a simultaneous command. Examples of simultaneous commands are a counter reset command and an input switching command. Examples of non-simultaneous commands are a reproduction command, a recording command, a fast forward command, and a rewind command.
FIG. 12 shows the implementation of the command communication technique described above. In this case, terminal (A) transmits to terminal (B) a plurality of non-simultaneous commands. Assuming that non-simultaneous command 1 is received first by terminal (B), terminal (B) begins executing non-simultaneous command 1. Terminal (B) also transmits to terminal (A) a busy signal 1, which indicates that terminal (B) is currently processing a command transmitted by terminal (A). If terminal (B) receives a second non-simultaneous command while it is processing non-simultaneous command 1, terminal (B) stops processing non-simultaneous command 1 and issues a reject signal 1, which indicates that terminal (B) has ceased processing non-simultaneous command 1. After transmitting reject signal 1, terminal (B) processes non-simultaneous command 2, and transmits busy signal 2. After terminal (B) has processed non-simultaneous command 2, terminal (B) transmits a complete signal 2 to terminal (A). Reference numeral "2" is used, instead of "1", because this complete signal corresponds to the completion of the second non-simultaneous command received at terminal (B). In this example, six packet signals were generated in order to execute one command.
In the case of FIG. 13, terminal (A) transmits to terminal (B) a non-simultaneous command 1 and a simultaneous command 2. After receiving non-simultaneous command 1 from terminal (A), terminal (B) transmits to terminal (A) a busy signal 1. After receiving simultaneous command 2 during the processing of non-simultaneous command 1, terminal (B) generates busy signal 2. In this situation, the non-simultaneous command 1 and simultaneous command 2 are performed concurrently. If non-simultaneous command 3 is then transmitted to terminal (B), terminal (B) halts the processing of non-simultaneous command 1 and issues a reject signal 1 to terminal (A). Terminal (B) then processes non-simultaneous command 3 and issues a busy signal 3 to terminal (A). After terminal (B) completes simultaneous command 2 and non-simultaneous command 3, it issues a complete signal 2 and a complete signal 3 to terminal (A). In such a case, nine packets are transmitted and received.
Thus, in the previously proposed system mentioned above, the reception of one command always causes the receiving terminal to issue a response thereto. Consequently, this method of communication results in an increased volume of data on the network.